Microlithography processes for making miniaturized electronic components, such as in the fabrication of computer chips and integrated circuits, involve using photoresists.
Generally, a coating or film of a photoresist is applied to a substrate material, such as a silicon wafer used for making integrated circuits. The substrate may contain any number of layers or devices thereon. The photoresist coated substrate is baked to evaporate any solvent in the photoresist composition and to fix the photoresist coating onto the substrate. The baked coated surface of the substrate is next subjected to selective radiation; that is, an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the photoresist coated surface. Types of radiation commonly used in microlithographic processes include visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy. After selective exposure, the photoresist coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist.
Especially with the trend towards miniaturization of semiconductor devices, there are problems that result from the back reflection of light from highly reflective substrates (back reflection from layers underneath the photoresist). Two deleterious effects of back reflectivity are thin film interference and reflective notching. Thin film interference results in a changes in critical linewidth dimensions caused by variations in the total light intensity in the photoresist film as the thickness of the photoresist changes. Reflective notching becomes severe as the photoresist is patterned over substrates containing topographical features, which tend to scatter light through the photoresist film, leading to linewidth variations, and in extreme cases, forming regions with complete photoresist loss.
Dyed photoresists have been utilized in an effort to solve these reflectivity problems. However, dyed photoresists only reduce reflectivity from the underlying substrate, they do not totally eliminate the reflectivity. Additional shortcomings associated with dyed photoresists are that dyed photoresists tend to cause reduction in the lithographic performance of the photoresist, the undesirable possible sublimation of the dye, and incompatibility of the dye in the photoresist films.
To prevent reflection of activating radiation into a photoresist coating, it is known to provide antireflection layers or antireflection coatings (ARCs) between a substrate and a photoresist layer. ARCs generally function by absorbing the radiation used for exposing the photoresist; that is, reducing reflectivity at the photoresist/underlying substrate interface. ARCs may comprise an absorbing dye dispersed in a polymer binder, though some polymers contain sufficient chromopores (or characteristics thereof) whereby a dye is not required. When used, the dye is selected to absorb and attenuate radiation at the wavelength used to expose the photoresist layer thus reducing the incidence of radiation reflected back into the photoresist layer. During conventional processing of an integrated circuit substrate coated with the combination of an ARC and a photoresist layer, the photoresist is patterned to form a mask defining a desired pattern.
To alter the underlying substrate, the ARC must be removed to bare the underlying substrate in a desired pattern. However, there are problems associated with removal of the ARC including incomplete removal, and/or undesirable damage, removal or interaction with the photoresist or underlying substrate. In addition to difficulties associated with removal of an ARC, other problems are often encountered when an ARC is used in combination with a photoresist coating. One such problem is that of selecting an antireflective coating that is compatible with the photoresist used. The ARC should be inert with respect to the photoresist coating while firmly bonding to the photoresist coating. Another problem is that of selecting an ARC that firmly bonds to the underlying substrate to avoid lift-off of the unremoved photoresist coating (from development) during processing of the underlying substrate. There are also problems associated with selecting ARCs having desirable properies at certain wavelengths that simultaneously possess the above mentioned desirable compatibility properties.
Titanium nitride is conventionally used as an ARC. However, when a titanium nitride ARC is used over a metal layer having a high reflectivity (for example, about 80%), reflectivity concerns continue to exist, especially at shorter wavelength irradiation (for example, about 248 nm or 365 nm). This is because the reflectivity of a titanium nitride ARC over a metal layer generally does not go below about 25%.
Moreover, with specific regard to deep ultraviolet (UV) photoresists, compatibility concerns exist with ARCs that contain nitrogen. This is because nitrogen deleteriously effects deep UV photoresists. In particular, nitrogen may migrate from the ARC and poison a deep UV photoresist by desensitizing them to UV light, thus inhibiting development of the photoresist. Desensitized deep UV photoresists exhibit extremely poor pattern definition after development.